Monolithic metallic structural elements having variable properties within the elements are very much in demand in the aeronautical industry. Structural elements are subjected to a wide variety of contradictory constraints that require particular choices about materials and working conditions. Such choices can lead to unsatisfactory compromises. Furthermore, replacement of long and expensive mechanical assembly steps by more economic integral machining steps of monolithic components is limited by the ability to obtain the most advantageous properties in each geometric zone of a monolithic element. Therefore it would be very useful to make monolithic structural elements having variable properties within the elements to obtain an optimum compromise of properties in each zone while benefiting from the economic advantages of integral machining processes. However, no process for manufacturing a monolithic metallic structural element with variable properties within the element has been industrialized due to cost and reliability problems.
Thus, several methods have been proposed in the prior art to make monolithic metallic structural elements with variable properties within each element.
A first proposed solution uses different heat treatments between the ends of the structural element at the time of artificial ageing.
FR 2 707 092 (Pechiney Rhenalu) describes a method of making structural work-hardened products with various continuously variable properties in at least one direction. This document achieves artificial ageing at a temperature T at one end and a temperature t at the other end in a special furnace comprising a hot chamber and a cold chamber connected through a heat pump.
WO 2005/098072 (Pechiney Rhenalu) describes a fabrication process in which at least one artificial ageing treatment step is carried out in a furnace with a controlled thermal profile comprising at least two zones or groups of zones Z1 and Z2 with initial temperatures T1 and T2 in which the length of the two zones is at least one meter.
These processes limit variations of properties to properties that can be modified compatibly during artificial ageing. These types of processes cannot be used for alloys without heat treatment. Similarly, for alloys in the 2XXX family for which many parts are sold in the T3 or T4 temper (not annealed), it is impossible to obtain elements with variable properties using this process.
US patent application 2003/226935 describes having a microstructure with increased amounts of fiber texture in a given plane perpendicular to the length an intra-rib area in order to reduce the rate of fatigue crack growth.
Another approach proposes to weld two parts made of different alloys before machining the resulting part. Although the material of the structural element obtained is continuous and its properties are variable within the element, it is not a monolithic structural element due to the welded zone.
PCT application WO 98/58759 (British Aerospace) describes a hybrid billet formed from a 2000 alloy and a 7000 alloy by friction-stir welding, from which a spar is machined. Patent application EP 1 547 720 A1 (Airbus UK) describes an assembly method by welding two parts typically obtained from different alloys to make a single structural part after machining for aeronautical applications such as a spar.
The problem is partly solved in the aeronautical industry by making local variations in the thickness of structural elements with homogenous properties within the elements so that they can resist local stresses. The thickness variation is usually obtained by assembly or by machining.
For example, CA 2 317 366 (Airbus Deutschland) describes the fabrication of fuselage elements by welding plates of different thicknesses. It would also be possible to obtain plates with variable thickness directly by rolling so as to prevent assembly steps and the associated technical and economic problems. Thickness variations would be possible in the longitudinal direction or the transverse direction (for example see R. Kopp, C. Wiedner and A. Meyer, International Sheet Metal Review, July/August 2005, p 20-24).
Furthermore, manufacturing of variable thickness plates has been envisaged by various methods, to solve other technical problems. Tailored blanks are also known in steelworks and provide a means of saving material during forming steps.
JP 11-192502 (Nippon Steel) describes a process for obtaining a steel blank for which the thickness and static mechanical characteristics vary across the width.
WO 00/21695 (Thyssen Krupp) describes a process for obtaining sections with a variable thickness along the rolling direction within a metallic blank, these sections having different mechanical properties.
Although it may be justified to save material, the modification in the geometry of plates has disadvantages in terms of fabrication, inspection and handling, and cannot provide a means for fast and direct transfer to existing processes used at aircraft manufacturers.
It is desired to develop an economical and controllable process for fabricating worked products and of monolithic structural elements made of an aluminium alloy, particularly for aeronautical construction, with usage properties that are variable within the element but having geometric characteristics identical to those of existing components. It is further desired to develop a process that varies the usage properties at various positions in the length of the structural elements but wherein the fabrication process does not require any artificial ageing.